Various methods of providing documents, such as negotiable instruments, with tamper evident systems to prevent alteration have been proposed. For example, U.S. Pat. No. 4,846,502 to J. Chang and T. Dimitriou discloses a tamper evident system in which a latent image message, such as the word "VOID", appears if someone tampers with the dollar amount on a negotiable instrument, such as a postal money order.
Likewise, systems have also been developed to prevent reproduction of documents by photocopying to reduce the incidence of fraud. However, in recent years advanced color copiers which are readily accessible to the general public can produce nearly exact duplicates of the original document. It is very difficult for the untrained person to distinguish the original document from an illicit reproduction. To prevent passing off of the reproduction as the original, efforts have been made to make the original document incapable of being copied or to incorporate authenticating systems into the document.
More recently, a system for document verification has been developed, which system is disclosed in co-pending U.S. application Ser. No. 07/987,710 to John C. H. Chang entitled "Heat Sensitive System and Use Thereof", and involves use of a heat-sensitive chromogenic system in the production of documents to prevent unauthorized or fraudulent use of a xerographic color copier for reproduction of negotiable instruments, such as checks, money orders and the like. When, for example, a check bearing the heat-sensitive chromogenic coating in a verification area thereof is presented for payment, the bank teller can simply apply heat to the verification area, and if a visible image symbol appears in the verification area, the teller will know that the check is an original. In such heat-sensitive record system, a heat-sensitive color developer is coated on the upper surface of the substrate. By subjecting the coating to a heat source or contacting the coating with a heated object, a visible colored mark is instantly produced. This system requires heat which melts or softens the heat-sensitive color developer without external pressure.
Laser printers in the form of desk models to computer-linked high speed models have been increasingly employed to upgrade office equipment for processing documents. Laser printers rely on toner to provide printed indicia on the documents. In order to adhere toner on the documents, heat is provided within the laser printers to melt and set toner on the surface of the documents. While there are many models of laser printer by numerous manufacturers, the heated surface of the fuser roll is at a temperature of, for example, between about 350.degree. F. to about 400.degree. F. When a document is fed into the laser printer, the temperature of the surface directly contacting the fuser roll could approach 400.degree. F. and the opposite surface could reach about 200.degree. F. Thus, a document bearing a heat-sensitive verification system could become prematurely colored, thereby destroying the verification system for such document.
Accordingly, a need still exists for a simple and efficient system which is able to accurately authenticate a document and distinguish it from a reproduction even after it has received printed indicia from a laser printer.